How does the negative air chamber affect the SID / NRS?- Mtbr.com

# Thread: How does the negative air chamber affect the SID / NRS?

1. ## How does the negative air chamber affect the SID / NRS?

I understand what the positive air chamber on a SID rear shock is for, but what exactly is the negative air chamber for and how does it affect the ride of an NRS? The shop manual says to pump the positive chamber to a pressure equal to your body weight, but to leave the negative at 50psi (regardless of rider weight). And in reading posts from other riders I noticed that some of you put more than 50psi into the negative air chamber. What does that do?

I'm 150lbs and have been riding my SID at 150/50 (+/-). I never messed around with the settings all that much because I am new to FS bikes and don't even know if I would know what is "right".

Thx...Doug

2. Well the positive chamber essentially sets the spring rate of the shock since the air in it is what's being compressed when the shock moves. The less air in the + chamber the weaker the spring and the less weight it'll support. What makes the shock move is a force against this spring. The negative chamber presses against it with an opposing force. Giant specs 50psi to provide a little force to counteract topping out at the end of the stroke. Putting more air in the - chamber adds to the force against the + chamber. The spring rate is essentially the same since the amount of air in the + chamber remains the same, but you have a "head start" towards moving that spring. The force of hitting a bump is essentially the same regardless of the shock, but the added force of the extra air will make the shock a little more active.
Bottom line is that you can add more pressure to the - chamber and see if that gives you a more plushness. It's still a low-volume shock and you most have more pressure in the + chamber for it to function properly. You'll also notice that the - chamber is smaller than the + chamber, that's why it takes fewer pumps to raise the pressure in it.

3. Basically how I understand is the more air in the -neg chamber the more it will respond to little bumps, and the less psi in the - chamber the stiffer it will feel.

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